Computational Insights into the Mechanisms of H2 Activation and H2/D2 Isotope Exchange by Dimolybdenum Tetrasulfide Complexes

Abstract

The mechanisms for H2 activation by [Cp*Mo]2(μ- S)2(μ-S2) (1-a, Cp* = pentamethylcyclopentadienyl) and its reaction product [Cp*Mo]2(μ-S)2(μ-SH)2 (2) have been investigated by DFT methods. The reaction of 1-a involves the homolytic addition of H2 to its μ-S ligands, followed by the cleavage of the S–S bond of the μ-S2 ligand in a subsequent step. Complex 2 can adopt five conformations that only differ in the stereochemistry of the μ-SH and μ-S ligands; although an isomer with adjacent μ-S ligands (2-a) is formed initially, it then isomerises into the experimentally observed 2-d. This species promotes H/ D scrambling in H2/D2 mixtures, and the mechanism of the process has also been studied. Notably, all of the computed pathways for the addition of D2 to 2-d present prohibitive barriers; instead, only those isomers with adjacent μ-S ligands are able to react further. The homolytic activation of D2 by these leads to isomers of [Cp2Mo2(μ-SH)2(μ-SD)2], the interconversion of which is the rate-determining step.